JACKSON HOLE -- In fall 2011, Jeff Burrell was sitting with his spotting scope at Trappers Point, a 6,000-year-old pronghorn migration corridor in Sublette County, watching the fleet-footed ungulates nervously approaching busy Highway 191.
The pronghorn clearly looked flummoxed and scared when faced with the obstacle on their path from Grand Teton National Park to winter range in south central Wyoming.
"They'd be about a mile away and you could see that they were on high alert," Burrell said, standing near the same vantage point this October.
The timid pronghorn would approach the road slowly, in groups, often being spooked into retreat.
"Sometimes, for the better part of the day, they'd go back and forth and back and forth and back and forth," said Burrell, a wildlife management specialist who works for the Wildlife Conservation Society.
Their intuition wasn't misguided. Combining pronghorn and mule deer, 702 animals were recorded killed on a 27-mile stretch of Highway 189/191 during a recent five-year period.
This year, Burrell said, the pronghorn are changing their behavior. The sharp-eyed animals still show some trepidation, but they now have a safe passage when they arrive at the barrier.
Aiding animals in crossing the deadly flow of traffic is now a $2.5 million, 150-foot-wide overpass.
To funnel 2,000 to 3,000 pronghorn and about 2,000 mule deer onto the Trappers Point overpass, plus another overpass and six underpasses, the Wyoming Department of Transportation installed about 31 miles of 8-foot-high fencing. This month marks the start of the first migration since the project was completed.
After a two-year construction process, the $9.7 million wildlife crossing project is now complete. Paid for and designed by WYDOT, it's the first overpass system in the world designed specifically for pronghorn antelope.
Before it was built, the animals would have to crawl under three strands of barbed wire before playing Frogger with traffic, often losing.
For about 400 Antilocapra Americana making the 100-mile-plus journey from their birthing and summer grounds in Jackson Hole, the overpass is a critical piece of protection. Their migration is the second longest by mammals in the Western Hemisphere.
Halfway through this year's migration, it's working great, said Renee Seidler, a Wildlife Conservation Society field biologist.
"There's hesitancy, nervousness, and you can definitely see the animals react to the structure," Seidler said, observing with Burrell from an overlook near Trappers Point. "There is that negative piece to it, but there's a huge positive piece. Best we know, 100 percent of the animals that are coming down here are crossing."
For individual animals, there's been a mixed bag of behavioral reactions to the new structure, located about 5 miles east of Daniel Junction.
"They're generally moving at a good pace headed for Trappers Point," Seidler said. "They go right to the fence on the west side, maybe a half-mile from the crossing."
After hitting the 8-foot fence, the pronghorn shift into a trot headed eastward toward the overpass, Seidler said.
"Almost always, they blow right by the old underpass," she said. "I have seen hundreds of pronghorn, and five of them have used the underpass. Everybody else is using the overpass."
Beginning last week, the first trickle of animals from Jackson Hole made it across the highway.
That's known because the Wildlife Conservation Society, Grand Teton National Park and the Wyoming Department of Game and Fish embarked on a telemetry study of pronghorn in Jackson Hole in 2010, outfitting 28 specimens with radio collars.